Electrical driver units generally comprise an electric motor and a power electronics circuit. The power electronics circuit draws power from a supply system at a fixed frequency and voltage and converts this power to produce a rotating field in a motor. The speed and torque of the motor are regulated by the power electronics circuit. The power electronics circuit generally comprises a servo amplifier or, in an unregulated drive system, a frequency converter. The servo amplifier and the frequency converter are normally together called an inverter and are actuated by means of the driver circuit provided in the power electronics circuit.
When electric drive units are being used, it is necessary for them to be immediately turned off and safely stopped in the event of faults or risks. That is to say that the motor must under no circumstances move on account of electrical actuation.
Normally, this is done by turning off the power supply for the power electronics circuit, as is known, by way of example, from the document BIA Report May 2003 “Sichere Antriebssteuerung mit Frequenzumrichtern” [Safe drive control using frequency converters], ISBN 3-88383-645-1 or from the document Antriebstechnik 33 (1994), No. 10, “Vermeidung von unerwartetem Anlauf bei stromrichtergespeisten Antrieben” [Avoiding unexpected starting in inverter-powered drives], Erwin Zinken, BIA St Augustin. This allows the motor to be reliably stopped, since no further power is supplied to the motor. However, when it is started again the entire power electronics circuit needs to be turned on again, which takes a considerable amount of time.
A further option is to isolate the motor from the power electronics circuit using an electromechanical switch, e.g. a contactor. However, the sudden switching can easily damage the power electronics circuit on account of overvoltages. In addition, the loading is also very high for the contactor, since high current levels need to be switched.
A further solution for safely turning off the rotating field is to suppress the ignition pulse. Ignition pulses are equivalent to control signals generated by the driver circuit in the power electronics circuit, which actuates the power stage in the power electronics circuit. The power stage has six electronic switches which are controlled by means of control signals, so that the internal DC voltage is converted into a three-phase alternating current. The ignition pulse can be suppressed in various ways. It is usual—as known from the aforementioned document BIA Report May 2003—to interrupt the supply voltage at the driver circuit. The voltage is usually turned off by a relay in the event of a fault. Safe stopping through ignition pulse suppression, i.e. by not producing the control signals, leaves all the other components in the power electronics circuits in a full standby state. For the ongoing application, it is thus possible to put the electrical drive system into the safe state and to activate it again without this being noticed. Delays when the driver circuit is turned on again do not arise in essence.
The switching of the supply voltage at the driver circuit has to date been effected by a mechanical relay, which is subject to wear. Such a mechanical switching relay does not allow the power electronics circuit to be designed for “single-fault safety”. “Single-fault safety” means that if a fault occurs in one of the safety-related components used the actuation of the motor is stopped immediately.